Is it just the upper that's different or is the lower lower different than a standard 5.56 AR. I have an opportunity to buy a preban Colt in 7.62 for a good price but probably would sell the upper and switch to 5.56 if possible.

I had a 7.62x39 setup. It's nice. A bit more recoil, but great for deer hunting. Only downside is the mags. Good mags are getting scarce. Your best bet is a "frankenmag", an AK magazine with an AR mag attached to the top. THey work pretty good.